The present invention is related to an improved method for inoculating cast iron late in the casting process and to an inoculant which affords more consistency in the inoculation of iron being cast. The inventive casting process, referred to as in the mold inoculation incorporates filtration and inoculation combining the advantages of both techniques for the manufacture of parts for which it is desired to obtain a structure free of iron carbides.
Cast iron is an extremely versatile engineered material comprising iron-carbon-silicon alloys that have been used in many commercial application manufacture of mechanical parts. The versatility of cast iron has led to the utilization of this material in many structural applications where the homogeneity and consistency of the iron will have a critical impact on the components performance. The casting of clean homogenous iron, specifically grey or ductile, is an essential step in the production of high quality engineered castings. Due to the importance of these cast items it is imperative that iron, specifically gray or ductile, be consistently cast with uniform morphology, with minimum included impurities and with properties that are reproducible.
Cast iron has an unusual metallurgical structure. Most metals form a single metallic crystalline structure during solidification. Cast iron, however, has a far more complex morphology during solidification. The crystalline phases that form during solidification of cast iron are dependent on the rate of solidification. Most engineered castings desire the formation of crystalline graphite within the iron matrix during solidification. If the cast iron solidifies too rapidly primary iron carbides can crystallize within the casting. Primary iron carbide is a hard brittle phase that makes the iron very difficult to machine and changes the physical and mechanical properties of the primary cast iron. Primary iron carbides are commonly referred to as xe2x80x9cchillxe2x80x9d. Carbon contained as iron carbides is generally considered to be detrimental in most iron castings whereas carbon present as graphite improves the physical and mechanical properties of cast iron. Carbon can crystallize as either iron carbide or graphite during solidification. The formation of either phase is driven by the rate of solidification and the degree of nucleation contained within the liquid iron. The rate of solidification is constrained by the geometry of the casting, the rate of heat extraction of the mold material and the amount of superheat of the iron contained when the metal entered the mold. The degree of nucleation is constrained by the metallurgical history of the molten iron. Carbon present as graphite is an advantageous form and persuading carbon to crystallize as graphite is an ongoing goal of standard foundry operations. Graphite can be present in several morphological forms including spherical, as is the case with ductile iron, and flake-like, which is the case with gray iron.
Standard foundry metallurgical practice includes inoculation wherein the nucleation and growth of graphite is encouraged at the expense of iron carbide formation. Preferential nucleation greatly enhances the mechanical and physical properties of the finished casting. Inoculation is typically done by addition of an inoculating agent to either the pouring ladle, the metal stream or within the mold. The inoculating agent is typically added to the pouring ladle by pouring the granulated inoculating agent into the ladle when the ladle is filled with liquid iron, whereas the inoculant is added to the metal stream by injecting or spraying a finely divided powder of the inoculating agent in the molten metal stream as the molten metal enters the mold. It is typically desirable to add the inoculating agent to the molten metal as late as possible to minimize fading. Insufficient or improper inoculation is constantly at the forefront of losses due to poor quality in a foundry operation.
It may be preferred for the formed graphite to be spheroidal, if a spheroidal graphite cast iron called xe2x80x9cSGxe2x80x9d or xe2x80x9cductilexe2x80x9d iron is required. Alternatively, a lamellar graphite cast iron is required for xe2x80x9cLGxe2x80x9d or xe2x80x9cgreyxe2x80x9d iron. The essential prior condition to be met is to prevent the formation of primary iron carbide.
To this end the liquid cast iron is subject before casting to an inoculation treatment, which will, as it cools, favour the appearance of graphite rather than that of primary iron carbide.
The inoculation treatment is therefore very important. It is in fact well known that inoculation, whatever the inoculants used, has on the liquid cast iron an effectiveness which reduces with time and which, generally, has already reduced by 50% after a few minutes. To obtain maximum effectiveness, one skilled in the art generally practises progressive inoculation, applying to this end several additions of inoculants at different stages of the development of the cast iron. The final addition is made in the mold as the molds are fed or even in the feed conduits of the molds by placing in the path of the liquid cast iron inserts constituted by an inoculant material. These inserts are generally used associated with a filter; in this case they generally have a perfectly defined shape in order to be able to be fixed in the filter, most often in an adapted cavity. These inserts of defined shape are known as xe2x80x9cpelletsxe2x80x9d or xe2x80x9cslugsxe2x80x9d. We will denote by the name xe2x80x9cfilter inoculant packagexe2x80x9d the unit constituted by the pellet and the filter.
There are two types of pellets. xe2x80x9cMoldedxe2x80x9d pellets are obtained by molding the molten inoculant. xe2x80x9cAgglomeratedxe2x80x9d pellets are obtained from a pressed powder with generally very little binding agent, or even without binding agent.
Commercial inoculants create nucleation sites by seeding the liquid iron with highly reactive elements. The reactive elements combine with oxygen and sulfur dissolved within the liquid iron and the resultant reaction products precipitate out of solution to form nucleation sites for graphite during solidification. These nucleation products continue to grow within the melt until the metal has completely solidified. These particulates must be within a narrow size range in order to nucleate graphite crystal growth. Thus seeding the metal with the reactive elements as close to solidification as possible increases the probability that the precipitated particles remain within the narrow size window necessary for nucleation of graphite crystals. Formation of crystalline graphite is contrary to the kinetically favored products. The critical parameters which affect inoculation are not understood and are still the subject of academic debate. The ability of a skilled artisan to predict, and therefore improve, inoculation efficiency is very much desired in the art.
Pellet inoculation, wherein the molten metal is exposed to a pellet just prior to a filter, is known wherein a base material comprising minor amounts of calcium, aluminum, and rare earths are used. As the casting proceeds the inoculation efficiency changes with time due to the kinetics associated with dissolution of the inoculating agent from the pellet. Further complicating the problems of inoculation is the realization that various pour volumes and times are desired for manufacturing different parts with different sizes. If long pour times are utilized, the method of ladle inoculation is undesirable due to fading of the inoculant in the ladle. If short pour times are utilized, the time may by insufficient to allow for the onset of inoculation by pellet inoculation. The properties which allow for effective inoculation in the metal stream are not well understood and typically a suitable working range is developed by experimentation at great cost and loss of material.
The Daussan patent FR 2,692,654 describes a filter inoculant package wherein the pellet is obtained by agglomeration of powder at 0.5 to 2 mm preferentially. The efficiency of this filter inoculant package is quite limited.
The Foseco Patent EP 0 234 825 describes a filter inoculant package wherein the inoculant is presented in the form of a powdery non-agglomerated powder enclosed in a plastic pouch. This unit is more complex to manufacture and employs non-agglomerated powder whose wettability with respect to the liquid cast iron is not always well controlled.
Efforts to alleviate the problem of effective inoculation are presented in the art with limited success. DE Patent Publication DE 43 18 309 A1, for example, incorporates an inoculating pellet into a depression of a filter. The filter, in a honeycomb, comprises pores of 1 to 8 mm. The effectiveness of this type of filter inoculant package proves in use to be restricted by that of the pellet employed. This accomplishes the goal of inoculating late in the process but does not mitigate the primary issue associated with the process dependent inoculation efficiency described above. The pellet/filter combination has been found to be of limited value to foundries since it does not provide any benefit, other than localizing the pellet.
U.S. Pat. No. 6,293,988 provides an inoculating agent which comprises oxysulphides. The advantage provided is the elimination of ferrosilicon as a carrier medium. The oxysulphide inoculating agent dissolves slowly and the rate of inoculation, particularly early in the pour, may be inconsistent and unpredictable. A slowly dissolving pellet is subject to problems associated with inefficient inoculation early in the pour even though the problem of fade may be mitigated to some extent.
Inoculants utilizing ferrosilicon carriers are known to dissolve very rapidly and therefore there use for ladle inoculation is widely accepted. The rapid dissolution rate has caused ferrosilicon carrier based inoculants to be overlooked in the art due to the understanding that the rapid rate of dissolution would cause the pellet to be dissolved prior to the end of the pour and therefore the inoculant would not be effective throughout the entirity of the pour. The rapid dissolution rate has made the ferrosilicon based inoculant difficult to control.
Prior to the present invention, artisans have been restricted to the use of ferrosilicon based inoculants in the ladle, injecting a stream of inoculant into flowing metal and non-ferrosilicon based inoculants as a pellet. Furthermore, the artisan has had to choose between fade, with ladle inoculation, ineffective inoculation early in a pour with pellet inoculation or the mechanical complexities associated with injection inoculation.
There has been a long standing desire in the art for an inoculating agent, and method of use, which insures consistent and predictable inoculation regardless of the rate at which molten metal is poured. Prior to the present invention this desire has not been met.
It is an object of the present invention to provide an inoculating pellet which consistently inoculates molten iron over a wide working range of pour times without fade or ineffective inoculation.
Another object of the invention is a filter inoculant package constituted by an agglomerated inoculant pellet and an associated filter, the respective characteristics of which have been adjusted to bring out a maximum synergy.
It is another object of the present invention to provide a system for inoculating molten iron which is easily controlled, does not limit the foundry operation and which can be utilized with virtually all existing foundry systems with minimal alteration of the physical structure and operational procedures.
It is another object of the present invention to provide an inoculation pellet which can be utilized to efficiently, and uniformly, inoculate molten iron over a wide range of approach velocities. This provides a particular advantage since the foundry can operate in a range which is dictated by manufacturing demands not limitations related to inoculation efficiency.
A particularly preferred embodiment is provided in a method for inoculating molten iron. The method comprises passing the molten iron through a filter assembly at an approach velocity of about 1 to about 60 cm/sec. The filter assembly comprises a filter element and an inoculation pellet in contact with the filter element. The pellet has an inoculant dissolution rate of at least 1 mg/sec. to no more than 320 mg/sec. and preferably comprises about 40-99.9%, by weight, carrier comprising ferrosilicon. The pellet further preferably comprises about 0.1-60%, by weight, at least one inoculating agent selected from rare earths or from a group consisting of cerium, strontium, zirconium, calcium, manganese, barium, bismuth, magnesium, titanium, aluminum, lanthanum and sulfur.
Another preferred embodiment is provided in an assembly comprising a filter and pellet for late inoculation of cast irons in their final filtration wherein said pellet is obtained by agglomeration of a powdered inoculant alloy and said filter is a refractory porous material, wherein said powdered inoculant of said pellet comprises a particle size distribution comprising 100%, by weight, less than 2 mm, 30-70%, by weight, between 50-250xcexc, and less than 25%, by weight, below 50xcexc and said filter only allows particles below 10xcexc to pass there through.
Another preferred embodiment is provided in a filter assembly comprising a porous filter and an inoculant pellet. The inoculant pellet comprises a carrier and inoculating element. The carrier comprises at least 30%, by weight ferrosilicon. The inoculant comprises at least one inoculating agent selected from rare earths or from a group consisting of cerium, strontium, zirconium, calcium, manganese, barium, bismuth, magnesium, titanium, aluminum, lanthanum and sulfur.
Yet another preferred embodiment is provided in a method for inoculating molten iron. The method comprises passing the molten iron through a filter assembly at a rate of about 1-60 cm/sec. The filter assembly comprises a filter element and an inoculation pellet in contact with the filter element. The inoculant pellet comprises a binder and about 0.1-60%, by weight, inoculant. The inoculant comprises at least one inoculating agent selected from rare earths or from a group consisting of cerium, strontium, zirconium, calcium, manganese, barium, bismuth, aluminum, lanthanum and sulfur. The pellet has an inoculant dissolution rate of at least about 1 mg/sec. to no more than about 320 mg/sec.
Yet another preferred embodiment is provided in a process for molding iron comprising the steps of
a) melting iron to form molten iron;
b) transporting the molten iron to a filter assembly wherein the filter assembly comprises a filter element and an inoculation pellet in contact with the filter element and wherein the inoculant pellet comprises a carrier and about 0.1-60%, by weight, active inoculant comprising at least one inoculating agent selected from the rare earths or from a group consisting of cerium, strontium, zirconium, calcium, manganese, barium, bismuth, magnesium, titanium, aluminum, lanthanum and sulfur and wherein the pellet has an inoculant dissolution rate of at least about 1 mg/sec. to no more than about 320 mg/sec;
c) passing the molten iron through the filter assembly at a rate of about 1 to about 60 cm/sec., measured at 30.25 cm2 cross section, to form inoculated filtered iron; transporting the inoculated filtered iron to a mold forming a molten shape; and
d) cooling the molten shape to form the molded iron.
Yet another preferred embodiment is provided in a pellet for inoculating iron in a mold. The pellet comprises about 40-99.9%, by weight, carrier and about 0.1-60%, by weight, inoculant. The carrier comprises at least about 30%, by weight, ferrosilicon. The inoculant comprises at least one inoculating agent selected from the rare earths or from a group consisting of cerium, strontium, zirconium, calcium, manganese, barium, bismuth, magnesium, titanium, aluminum, lanthanum and sulfur. The pellet has an inoculant dissolution rate of at least about 2 to about 250 mg/sec. measured at 15 cm/sec approach velocity with a 30.25 cm2 iron flow.